Photoluminescent Quantum Dots (QDs) are an important research field at the frontier of science. There is keen interest in both the fundamental understanding of their photophysical properties and in their promising bio-oriented and energy-oriented applications.
Semiconductor QDs are single nanocrystals, typically spherical in shape, with diameters of a few nanometers (nm). For bulk semi-conductor materials, the absorption of a photon with energy above the semi-conductor bandgap energy results in the creation of an electron-hole pair called an exciton. When nanocrystals are smaller or comparable to their Bohr excitons (a few nanometers (nm)), their bandgap energy increases with energy levels quantized: the bandgap energy value is directly related to their sizes. Such a size-related effect is called quantum confinement; hence, the spherical nanocrystals are termed as “quantum dots”. In addition to the bandgap, quantum confinement effects lead to physical properties, including electronic and magnetic properties of QDs that are substantially different from those of bulk materials. It is known to produce semiconductor QDs by various methods. Generally QDs need to be isolated from each other so that they do not agglomerate and accrete, as accreted agglomerates of QDs become bulk crystalline material that does not exhibit the useful properties of QDs. It is known to isolate QDs from each other: 1—by forming QDs on substrates, 2—by coating individual QDs with inert layers, or 3—by capping the QDs with surface ligands to produce what are known as colloidal semiconductor QDs.
The sizes of colloidal semiconductor nanocrystals can be controlled by parameters of the process by which they are produced. It is known to produce colloidal nanocrystals by wet chemistry, so that the semiconductor nanocrystals are bound to one or more ligands which serve to isolate the nanocrystals. Methods of producing colloidal semiconductor nanocrystals by known wet-chemistry synthetic methods yield a various ensembles of semiconductor nanocrystals of various purities, separated by one or more known ligands. For example, approaches with hot injection, and reverse micelle reactions are known.
As noted by Y Charles Cao in Angewandte Chimie Int. Ed. 2005, 44, 6712-6715, and in WO06023206, the most successful and widely used nanocrystal synthesis method relies on rapid precursor injection, but unfortunately these are not readily industrially scaled. The process for controlling crystal growth requires very short periods and excellent thermal controls to produce quality nanocrystals. Cao proposes a technique that is based on a purified cadmium myristate precursor, and only uses an acid for stabilizing the growth of nanocrystals after a point.
There have been a great number of methods of synthesizing colloidal semiconductor nanocrystals. One example is taught by Yu and Peng in Angewandte Chimie Int. Ed. 2002, 41, 13, 2368-2371. In this example, as in many others that involve ligand forming acids having carboxylic groups, Cd precursors are produced that are solubilized by virtue of the ligand forming acids. Specifically CdO is mixed with oleic acid, whereby the oxygen is stripped from the Cd and replaced by a pair of oleic acid molecules to produce the precursor, in an ODE reaction medium. If the CdO is not dissociated, it will not be suspended in the ODE. Accordingly this method requires a higher acid to CdO molar ratio. It goes without saying that CdO precipitate will interfere with nanocrystal synthesis. This method is just one example of hot injection reactions that requires heating of the ODE and Cd precursor to 300° C. followed by injection and rapid crystal growth (on the order of tens of seconds) at a lower temperature (250° C.). In short their method would not work if a lower acid to Cd molar ratio were provided.
Furthermore the 2:1 or greater acid:Cd molar ratio required in such reactions (molar ratios of 3:1-210:1 were used) have the effect of making the precursor highly soluble, as both binding sites of the Cd are occupied by long chain (oleic) acids. Moreover, excess acid in the ODE improves the solvent's ability to suspend the precursors. All of this makes for fast precipitation reaction using very different precipitation dynamics from those used in the present invention. This is characteristic of such high temperature injection methods.
There are several properties of semiconductor nanocrystals that make them of interest, including photoluminescence. Adv. Mater. 1999, 11, 1243 “Photoluminescence from Single Semiconductor Nanostructures” by Moungi G. Bawendi, et al. notes “Size-dependent optical properties with band edge absorption and emission wavelengths that are tunable across the visible range (˜400±700 nm) make CdSe nanocrystals of particular interest for the study of fundamental physics as well as potential optoelectronic device application . . . . Variations in size and shape within ensemble samples can result in extensive inhomogeneous spectral broadening . . . . The result is a loss of spectral information in ensemble samples.” In this paper, single semiconductor nanocrystals were experimented with, and the inhomogeneous spectral broadening was found to be 80 meV at a low temperature (10 K). The problem of inhomogeneous spectral broadening is well known in the art.
One reason for desiring a narrow linewidth is to provide specific, selective response of a probe, which may be useful in biomedical probe applications, bar code applications, and other molecular labeling analyses. Another reason is for laser applications where narrow emission spectra provide high energy density at specific wavelengths and therefore provide more efficient conversion of pump power into a single mode emission. It may further be desired to use such narrow linewidth emitters for standards and references. They might also be used in display devices.
For example, International patent application WO 03/012006 to Peng et al., entitled Colloidal Nanocrystals With High Photoluminescence Quantum Yields asserts the need for colloidal nanocrystal production methods that allow for the manipulation of the purity of the emission color, by controlling the full width at half maximum (FWHM) of the nanocrystal PL peak. Peng et al state that it is desirable to develop methods that provide emission peaks sufficiently sharp so as to approach those observed by single dot spectroscopy (in the 20 nm range). Peng et al. also indicates that the bandwidth of known CdSe colloidal solutions have a peak linewidths around 27-40 nm wide (FWHM), and that single dot spectroscopy indicates that the individual peak linewidths are less than 20 nm. The narrowest CdSe nanocrystal ensembles produced by Peng et al. have peaks 23-24 nm FWHM.
Naturally it would be highly desirable to provide an ensemble of nanocrystals that have nominally a same size, so that the ensemble exhibits single size optical properties. While this might seem impossible given the thermodynamics of the chemical processes used to produce colloidal semiconductor nanocrystals, there have been some reports of “magic sized” colloidal semiconductor nanocrystals. The theory behind magic sized nanocrystals is that some structural features of the nanocrystals admits of preferential formation of certain sizes in analogy to gold and carbon which have known magic sizes. The challenge is to provide an environment that permits such preferential formation.
Unfortunately “magic size” properties of semiconductor materials are not well understood. The physics of the nanocrystals structurally, and accordingly the properties that they exhibit are still in question. As noted above by Peng et al. some researchers have believed that single sized nanocrystals would have a line width of about 20 nm. Schlegel et al. (G. Schlegel, et al. Phys. Rev. Lett. 2002, 88(13), 137401) reported that individual nanocrystals (ZnS-covered CdSe QDs) have line widths of 50 meV, or roughly 10 nm. Magic size nanocrystals are widely presumed to occur only in the smallest of QD sizes, which is natural given that the other existing magic size nanocrystals such as Gold (55), hot Sodiums (8, 20, 40, and 58) and Carbons (60 and 70) have fewer than 100 atoms.
As early as 1998, Ptatschek et al. (Ptatschek, V. et al. Ber. Bunsenges. Phys. Chem. 1998, 102, 85-95) reported magic-sized CdSe clusters, obtained at room temperature by cluster chemistry. These clusters exhibited sharp HOMO-LUMO absorption peaks at 280, 360, and 410 nm, corresponding to gyration sizes of 0.42, 0.85, and 1.7 nm suggested by small-angle X-ray scattering (SAXS), respectively. A composition of these clusters was found to be Cd34Se19L37.5 (L=ligands). Structurally a Koch pyramid structure with the lateral length of 1.7 nm was proposed for the cluster exhibiting its absorption peak at 410 nm. No photoemission is associated with the HOMO-LUMO peaks, and so these clusters are not bandgap photoluminescent. The linewidths of the HOMO-LUMO absorption peaks are greater than 20 nm.
In 2001 (Soloviev, V. N. et al. J. Am. Chem. Soc. 2001, 123, 2354-2364), a series of CdSe cluster molecules, synthesized at room temperature by cluster chemistry, was described to cover a size range of 0.7-2 nm. Single-crystal X-ray diffraction and elemental analysis showed that the clusters have 4, 8, 10, 17, or 32 Cd atoms capped with selenophenol capping ligands, with a combination of adamanthane and barylene-like cages, which are the building blocks of the zinc-blende and wurtzite bulk CdSe. Photoluminescent excitation (PLE) performed at 8K showed an exciton absorption peak of the 32-Cd cluster at 374 nm. No bandgap photoluminescence is observed.
With a reverse-micelle approach, in 2004 (Kasuya, A. et al. Nat. Mater. 2004, 3, 99-102), Kasuya et al. reported a (CdSe)n nanocrystal ensemble exhibiting a sharp absorption peak at 415 nm (at the lowest energy). These nanocrystals were characterized as having n=33 or 34 by time-of-flight mass spectra and a diameter of 1.5 nm by atomic force microscopy. With first-principles calculations, they were imaged with a core-cage structure, a three-dimensional network consisting of a core of (CdSe)5˜6 and cages of (CdSe)28. The width of the band edge absorption band of the (CdSe)34 single-sized ensemble reported by Kasuya et al. (Kasuya, A. et al., Nature Materials 2004, 3, 99) was estimated from their FIG. 2 to be ˜150 meV, which is greater than 20 nm.
16 a hot-injection approach in 2005 (Bowers, M. J. et al. J. Am. Chem. Soc. 2005, 127, 15378-15379), Bowers et al. reported producing a CdSe magic-sized nanocrystal ensemble exhibiting a narrow (FWHM>20 nm) exciton absorption spectrum peaking at 414 nm. This ensemble was synthesized 16 the injection of a Se-precursor solution into a Cd-precursor solution at 330° C., followed by a short growth period of 2-10 s at lower temperature. The nanocrystals ensemble showed no bandgap photoemission.
At the end of 2007, a CdSe MSN ensemble exhibiting a narrow (FWHM>20 nm) exciton absorption spectrum peaking at 414 nm was reported (Dai, Q. et al. Nanotechnology 2007, 18, 405603). The ensemble was obtained 16 a hot-injection approach involving the injection of a Se-precursor solution into a Cd precursor solution at 220° C., followed by a growth at 190° C. The nanocrystals ensemble showed no bandgap photoemission. While it is difficult to assess the FWHM for the curves given, the linewidth of the absorption spectrum peak at 414 nm is greater than 15 nm.
In all of the above descriptions of MSNs produced by various methods (reverse micelle, hot injection, and cluster chemistry), nanocrystals are produced, but in no case did the nanocyrstals exhibit band gap photoluminescence, and in no case did the ensembles exhibit an absorption spectrum having a peak as narrow as 10 nm, or at a wavelength above 415 nm.
At the beginning of 2007, Kudera et al. reported several families of CdSe “MSNs” with their first absorption peaks at 330, 350-360, 384, 406, 431, and 447 nm (Kudera, S. et al. Adv. Mater. 2007, 19, 548-552). The families were produced by the injection of a Se-precursor solution into a Cd-precursor solution at 80° C., followed by a growth period spanning from 3 min to several hours at 80° C. The ligands were in the form of amines. Kudera et al. reports that these families developed 16 sequential growth: relatively small families progressively evolved into relatively larger ones.
These families exhibit peaks having absorption linewidths (FWHM) in the neighbourhood of 20 nm. If the intrinsic linewidth of a QD were around 20 nm as suggested by some researchers, these might be taken to be MSNs, however it does not appear to be so. It is unlikely that the CdSe QDs produced are exclusively MSNs given the linewidths shown in the paper.
According to Kudera et al., the families produced exhibit considerable photoluminescent emission from trap states, and band-edge emission that was only clearly visible at very high dilutions. No band-edge emission was demonstrated in the results. Furthermore as agglomeration appears to have occurred subsequent to the size-selective precipitation process used to substantially isolate one family from the produced ensemble containing other QDs, it is far from clear to one skilled in the art what was observed. It might have been trap state emission that was observed. In any case, Kudera et al. does not show QDs that demonstrate band gap photoluminescence, and on the evidence of the paper no peak having a linewidth less than 20 nm FWHM is presented.
Moreover the absorption peaks of the families taught by Kudera et al. do not coincide with any of the stable families of bandgap photoluminescent disclosed herein.
Kudera et al. tacitly endorses the belief that MSNs are smaller than QDs. In all of the above descriptions it is clear that the belief in the art is that MSNs are small quantum dot phenomena (clusters—small nanocrystals). That is, while QDs are generally 1-10 nm in diameter, CdSe MSNs are smaller than 2 nm in diameter, and correspondingly have absorption peak wavelengths in lower than 450 nm. Reported CdS MSNs have absorption peak wavelengths of (˜305 nm see Yu and Peng above). Reportedly, CdTe MSNs have absorption spectral peaks broader than 20 nm centered around 450 nm and lower (Phys. Chem. Chem. Phys., 2003, 5, 1253-1258, and J Phys. Chem. C, 2007, 111, 14977-14983). None of these are bandgap photoluminescent.
There therefore remains a need for a method of synthesizing nanocrystals that provides an ensemble of colloidal semiconductor nanocrystals. In particular there remains a need for a method of synthesizing colloidal semiconductor nanocrystals that provides long growth/annealing periods. There remains a need for a low cost method of synthesizing high quality colloidal semiconductor nanocrystals that does not require hot injection, and so is readily industrially scaled-up. The need for methods of synthesizing colloidal semiconductor nanocrystals having narrow linewidths and bandgap photoluminescence is high.
There is a need for ensembles of colloidal semiconductor nanocrystals that exhibit narrow absorption peaks, less than or near 10 nm at FWHM, especially those with absorption peaks at or above 450 nm, since larger size colloidal semiconductor nanocrystals families are more stable.
There is also a need for ensembles of colloidal semiconductor nanocrystals of ternary structures, such as CdSeTe, especially when they exhibit high stability.
The most potent wavelength range for controlling melatonin production is 446 nm-477 nm. Therefore, there is a further need for emitters that have the potential application in the therapeutic use of light for treating winter depression and circadian disorders. There is also a need for emitters having high quantum yield.
There remains a need for a colloidal semiconductor nanocrystal that has improved stability and can be preferentially produced, with absorption peaks at or above 450 nm. Colloidal semiconductor nanocrystals are also desired that are bandgap photoluminescent, especially at or above 450 nm. It is noted that peaks in this neighbourhood are blue in colour or redder.